1. Field of the Invention
The present invention relates to magnetic powder cores and to methods for making the same. In particular, the present invention relates to a low-coercive-force, low-loss magnetic powder core and a method for making the same. The present invention also relates to switching power supplies, various converter circuits, and active filters. Furthermore, the present invention relates to filters and amplifying devices, and particularly, relates to a low-loss filter outputting less distorted waveforms.
2. Description of the Related Art
As magnetic cores used in core components, such as transformer cores for switching power supplies and smoothing choke cores, which require a constant permeability up to the high frequency region, ferrite closed-magnetic-circuit cores, ferrite gapped cores, and amorphous-alloy-tape-wound cores provided with gaps have been proposed. Also, magnetic powder cores formed by compacting a mixture of a powder, such as carbonyl iron, permalloy, or sendust, and an insulating material have been proposed.
Ferrite sintered magnetic cores exhibit low core loss, but simultaneously exhibit small saturation magnetic flux densities. Thus, in ferrite closed-magnetic-circuit cores and ferrite gapped cores, a leakage magnetic flux from the gap section adversely affects peripheral electric circuits. Magnetic powder cores using powders of carbonyl iron, permalloy, and sendust have the disadvantage of large core loss, although the cores exhibit higher saturation magnetic flux densities compared to ferrite magnetic cores.
In recent years, development of electronic devices has advanced with an increase in the use thereof. In particular, the weight of the development was shifted toward reducing heat dissipation by reducing the size of the electronic devices and reducing the power loss. In order to achieve these aims, switching power supplies, various DC/DC converter circuits, and active filters have been improved. These devices use various types of magnetic elements having magnetic cores. Ferrite is mainly used for the magnetic cores. In some cases, carbonyl iron magnetic cores, FeAlSi-alloy magnetic powder cores, and FeNi-alloy magnetic powder cores are also used.
A ferrite magnetic core is generally provided with a gap to prevent magnetic saturation. A leakage magnetic flux from the gap will adversely affect peripheral circuits. On the other hand, a NiZn ferrite core exhibits a large core loss, resulting in high heat dissipation from a device using this core. A carbonyl magnetic powder core exhibits an extremely large core loss, resulting in significantly high heat dissipation compared to ferrite magnetic cores. In addition, in a FeAlSi-alloy magnetic powder core and a FeNi-alloy magnetic powder core, the core loss thereof is lower than that of the carbonyl iron magnetic powder core, but still does not reach required levels.
Low-pass filters have been used for smoothing the pulse shape output from impulse modulation amplifiers. The requirements for low-pass filters are low loss and less distortion of smoothed waveforms. A low-pass filter is generally provided with a capacitor and an inductor composed of a coil with a magnetic core. Achievement of these requirements strongly depends on properties of the magnetic core constituting the inductor. Thus, conventional low-pass filters use amorphous magnetic cores provided with gaps, ferrite cores provided with gaps, or carbonyl iron gap-free magnetic powder cores.
However, in filters using amorphous magnetic cores provided with gaps or ferrite cores provided with gaps, leakage magnetic fields from the gaps may adversely affect peripheral elements and circuits, resulting in decreased stability in the entire circuits including the filters and generation of noise. Moreover, in these filters, the amplitude permeability varies with changes in the magnetic field and exhibits a large rate of change. When a pulsed current causing a large change in magnetic field is smoothed, the waveform will be significantly distorted.
In the carbonyl iron gap-free magnetic powder cores, the dependence of the amplitude permeability on the magnetic field is constant, and the waveform is not distorted. However, the carbonyl iron gap-free magnetic powder cores dissipate a significant amount of heat due to large core loss.
The large core loss in conventional magnetic powder cores is due to large core loss of the magnetic materials themselves used for the magnetic powder and insufficient relaxation of stress which is applied during compacting of the magnetic powder cores.